Blackheads are a mild form of acne that appear as unsightly, open pores that look darker than the skin surrounding them. They get their dark appearance from a skin pigment called melanin, which oxidizes and turns black when it's exposed to the air. Blackheads aren't caused by dirt, but by sebum (oil) and dead skin cells blocking the pore. If the pore remains open, it becomes a blackhead; if it's completely blocked and closed, it turns into a whitehead.
Pimples are raised red spots with a white center that develop when blocked hair follicles become inflamed or infected with bacteria. Blockages and inflammation that develop deep inside hair follicles produce cystlike lumps beneath the surface of your skin. Other pores in your skin, which are the openings of the sweat glands, aren't usually involved in acne.